1. Field of the Invention
This invention relates to a method and a unit for treating wastewater with microorganisms, and specifically to a method and a unit for wastewater treatment using a contact oxidation process.
2. Description of the Prior Art
Known techniques for wastewater treatment by the action of microorganisms include, for example, a conventional activated sludge process, a contact stabilization process, a step aeration process, a high-rate aeration process, a rotating biological contactor process, a trickling filter process, and a contact oxidation process (e.g., as disclosed in A Critical Review of Current Municipal Wastewater Treatment Technology. Phase I-Biological Treatment Methods, Pacific Northwest Laboratories, Battelle Memorial Institute, July 1973).
In these microbial wastewater treatment processes, a large variety of microorganism in large numbers become a mass and resemble mud. Hence, the mass of microorganisms is termed "sludge". The above-described wastewater treatment processes using microorganisms can be roughly classified as a flotation process and fixed bed process depending on whether the sludge is suspended in an aeration tank or is retained in a fixed bed. The conventional activated sludge process, contact stabilization process and high-rate aeration process are considered flotation processes, and the contact oxidation process and trickling filter process are considered fixed bed processes.
The method in accordance with this invention can be described as a contact oxidation method. Generally, the contact oxidation process which employs an air diffuser or a mechanical aerator is superior in treating ability and treating efficiency to a mere fixed bed process, for example, the rotating biological contactor, for example, the process (e.g., as disclosed, for example, in Sadao Kojima, Yosui To Haisui (Industrial Water and Waste Water), 14, p. 960, (1972) and Y. Maeda, Journal of Fermentation Technology, 53, p. 875 (1975)).
The contact oxidation process is a process by which a certain supporting medium is placed in an aeration tank to grow microorganisms on the surface of the medium or both on the surface of and in the interstices of the medium, and by aeration aerobic microorganisms on the surface of the supporting medium and anaerobic microorganisms in the interstices thereof act to remove organic polluting matter in wastewater. This is also called a fixed bed aeration method. Various methods have been suggested and employed as to the structure, material and stacking of the supporting media to be disposed in the aeration tank, the manner of arrangement of the supporting media, etc. For example, a plastic cylinder having an inside diameter of about 25 to 75 mm, or a honeycomb-shaped block (i.e., a combination of a plurality of hexagonal cells) having an inside diameter of about 10 to 75 mm is suspended in an aeration tank, microorganisms are grown on the surface thereof, and aeration is performed (e.g., as disclosed in Sadao Kojima, Collection of Informative Materials for Techniques of Producing and Utilizing Regenerated Water, Fuji Technosystem Press (1974)). However, in wastewater treatment using such contact oxidation processes, microorganisms retained on the surface and inside of the supporting media frequently are sloughed off in the form of a mass or sheet, and cause a clogging of the inside of the cylindrical or honeycomb-shaped supporting ("sloughing-clogging phenomenon"). The occurrence of the sloughing-clogging phenomenon markedly reduces the water flow in the supporting media, namely at the fixed bed portion with microorganisms retained thereon. The contact oxidation process simultaneously utilizes the activities of aerobic microorganisms and anaerobic microorganisms. Generally, however, the action of removing polluting matter in wastewater is due primarily to aerobic microorganisms, and secondarily to anaerobic microorganisms. Thus, when the water flow is reduced by the sloughing-clogging phenomenon, polluting matter and dissolved oxygen in the waste water cannot be carried to a location where microorganisms having a treating ability are present. This immediately results in reduced treating ability or treating efficiency.
A method has also been suggested which involves treating organic polluting matter in wastewater using activated sludge which has been retained on and cultivated and filled in the open cells of pieces, usually with a size of 1 to 100 cm.sup.3, of a spongy resin such as a polyurethane foam or a polyethylene foam, and thus has been converted to large particles having the size of the resin fragments (e.g., as disclosed in Japanese Patent Application (OPI) No. 64261/74). This method using spongy resin pieces, however, has the defect that the lifetime of the resin pieces as supporting media is about 2 years at the longest and it is difficult for them to hold microorganisms for long periods of time, and that, since many pieces float in the aeration tank, it is difficult to examine all of the individual pieces and exchange only those which have been worn out.
In microbial wastewater treatment, the microorganisms serve to remove organic polluting matter in the wastewater by absorbing, or oxidatively decomposing the matter. In the meantime, the microorganisms grow utilizing the polluting matter as a nutrient source, and frequently increase to a number more than necessary for the purpose of wastewater treatment. The excess is usually called excess sludge, and is discharged from the wastewater treatment system.
In almost all of the wastewater treatment processes using the activities of microorganisms, excess sludge occurs more or less, and, as a secondary problem of sewage disposal, large equipment, much labor and high costs must go into the treatment of the excess sludge. Thus, this constitutes one of the great defects of microbial wastewater treatment processes.
In an attempt to remedy this defect, methods have been employed in which the air flow rate is increased, or aeration is performed using highly pure oxygen, or the excess sludge is stored in a separate excess sludge tank and aerated for long periods of time (e.g., as disclosed in J. E. Ball, and M. J. Humenick "High-Purity Oxygen in Biological Treatment of Municipal Wastewater", Journal of Water Pollution Control Federation, 44, 65 (1972), T. D. Reynolds, "Aerobic Digestion of Waste Activated Sludge" Water Sewage Works, pp. 37 (Feb., 1967), etc.). These methods based on oxidation under aerobic conditions have the advantage of decreasing the excess sludge, but on the other hand, involve increased costs for power used to supply air or produce highly pure oxygen.
Generally, the fixed bed processes such as the rotating biological contactor process and contact oxidation process yield somewhat smaller amounts of excess sludge than the flotation processes such as the conventional activated sludge process or the contact stabilization process because, although the surface of the sludge adhering to the fixed bed is aerobic, the inside of the sludge is anaerobic, and the action of enzymes produced by the anaerobic microorganisms causes a digestion of the microbial cells. However, the amount of excess sludge produced is not small, and the fixed bed processes, too, are not free from the problem of treating the excess sludge.